Impacts – IX – Sea Level 4 – Sinking Megacities

So the cost of sea level rise for 2100 in the US seems to be a close to zero cost problem.

Probably the provocative way I wrote the conclusion confused some people. I should have said that it was a very expensive problem. But that it wasn’t a problem that society should pay for, given that anyone moving to the coast since 2005 at the latest would have known that future sea level was considered to be a major problem. By 2100 the youngest people still living right on the sea front, who bought property there before 2005, would be at least 115 years old.

The idea is that “externalities” as economists call them should be paid by the creators of the problem, not the people that incur the problem. In this case, the “victims” are people who ignored the evidence and moved to the coast anyway. Are they still victims? That was my point.

Well, what about outside the US?

Some mega cities have huge problems. Here is Nicholls 2011:

Coastal areas constitute important habitats, and they contain a large and growing population, much of it located in economic centers such as London, New York, Tokyo, Shanghai, Mumbai, and Lagos. The range of coastal hazards includes climate-induced sea level rise, a long-term threat that demands broad response.

Global sea levels rose 17 cm through the twentieth century, and are likely to rise more rapidly through the twenty-first century when a rise of more than 1 m is possible.

In some locations, these changes may be exacerbated by

(1) increases in storminess due to climate change, although this scenario is less certain
(2) widespread human-induced subsidence due to ground fluid withdrawal from, and drainage of, susceptible soils, especially in deltas.

Subsidence?

Over the twentieth century, the parts of Tokyo and Osaka built on deltaic areas subsided up to 5 m and 3 m, respectively, a large part of Shanghai subsided up to 3 m, and Bangkok subsided up to 2 m.

This human-induced subsidence can be mitigated by stopping shallow, subsurface fluid withdrawals and managing water levels, but natural “background” rates of subsidence will continue, and RSLR will still exceed global trends in these areas. A combination of policies to mitigate subsidence has been instituted in the four delta cities mentioned above, combined with improved flood defenses and pumped drainage systems designed to avoid submergence and/ or frequent flooding.

In contrast, Jakarta and Metro Manila are subsiding significantly, with maximum subsidence of 4 m and 1 m to date, respectively (e.g., Rodolfo and Siringan, 2006; Ward et al., 2011), but little systematic policy response is in place in either city, and future flooding problems are anticipated.

Subsidence graphic:

From Nicholls 2011

Figure 1

To put these figures in context, sea level rise from 1900-2000 was about 0.2m and according to the latest IPCC report the forecast of sea level rise by 2100 might be around an additional 0.5m (for RCP 6.0, see earlier article). In the light of the idea that global society should pay for problems to people caused by global society, perhaps the problems of Shanghai, Bangkok and other sinking cities are not global problems?

Here is Wang et al from 2012:

Shanghai is low-lying, with an elevation of 3–4 m. A quarter of the area lies below 3 m. The city’s flood-control walls are currently more than 6 m high. However, given the trend of sea level rise and land subsidence, this is inadequate. Shanghai is frequently affected by extreme tropical storm surges. The risk of flooding from overtopping is considerable..

..From 1921 to 1965, the average cumulative subsidence of the city center was 1.76 m, with a maximum of 2.63 m. From 1966 to 1985, a monitoring network was established and subsidence was mitigated through artificial recharge. Land subsidence was stabilized at an average of 0.9 mm/year. As a result of rapid urban development and large-scale construction projects between 1986 and 1997, subsidence of the downtown area increased rapidly, at an average rate of 10.2 mm/year..

..In 2100, sea level rise and land subsidence will be far greater than before. Sea level rise is estimated to be 43 cm, while land subsidence is estimated to be 3–229 cm, and neotectonic subsidence is estimated to be 14 cm. Flooding will be severe in 2100 (Fig. 8).

[Note I changed the data in the last paragraph cited to round numbers in cm from their values quoted to 0.01cm – for example, 43cm instead of the paper’s values of 43.31 etc].

So for Shanghai at least global sea level rise is not really the problem.

Given that I don’t pay much attention to media outlets I probably missed the big Marches against Ground Water Depletion Slightly Accentuating Global Warming’s Sea Level Rise in Threatened Megacities.

As with the USA data the question of increased storm surges accentuating global sea level rise is still on the agenda (i.e., has not yet been discussed).

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14 Responses

CO2 abatement is a global solution to local problems. But it will only partly reduce sea level rise, which is not a main part of the local problems. So cities like Shanghai will still have to find local solutions.

Other low lying cities, like Amsterdam and New Orleans, have dealt with such problems using much less sophisticated technology than is available today. And Chicago simply raised the entire city by about 2 meters, using mid-19th century technology, mostly powered by human muscles: https://en.wikipedia.org/wiki/Raising_of_Chicago
Talk about “just do it!”

I Google Earthed Jakarta to see if any effects of the subsistence . Clearly not a pervasive problem there . And they are actually dredging new land from the ocean like so many places , including the area of Manhattan by South St where I lived , have done over the centuries . I think one trip to The Netherlands should cure anybody of abnormal fear of the constant 30cm per century rise in sea level we’ve experienced for a long time .

SoD
“The idea is that “externalities” as economists call them should be paid by the creators of the problem, not the people that incur the problem. In this case, the “victims” are people who ignored the evidence and moved to the coast anyway. Are they still victims? That was my point.”

You’re ignoring all the public infrastructure that would need replacing as well as this strange idea that impacts only start in 2100.

There’s also a host of other issues, such as seawater intrusion to local groundwater supplies.

It’s a hugely complex issue and you have presented a very trivial analysis.

Agree about groundwater abstraction leading to subsidence, and this is a critical issue (not just around groundwater supplies), but Libertarians will hate the solution – it’s called Regulation.

Yes SoD, We could fix this problem very easily in the US by eliminating Federal flood insurance. Anyone who builds in a flood zone (such as within 20 feet of mean sea level in coastal areas) is on their own. That would induce more sane behavior. Generally the lifetime of a home is roughly 40- 100 years depending on initial construction quality. As sea level rises, those too close will drop in vale and eventually be torn down or swept away.

∞ Our nation has over 3.5 million miles of rivers and streams.
∞ About seven percent of the land area (around 175 million acres) is subject to periodic
flooding.
∞ These “floodplains” contain an abundance of water, living, and cultural resources
(described in a subsequent chapter) of immense value.
∞ Most wetlands are located in floodplains (103 million acres of wetlands), comprising about five percent of the nation’s land area.
∞ There are approximately 17,000 to 18,000 flood-prone communities.
∞ Fifteen percent of urban areas are subject to flooding. Over half of urban floodplains have been already developed.
∞ Floodplains contain approximately 10 million households and $800 to $900 billion in property subject to flood risk.
∞ Floods are the nation’s greatest natural disaster, causing an estimated $3 to $5 billion in average annual damages (actual figures have never been compiled).
∞ Annual growth in coastal and riverine floodplains is roughly twice that of the country as a whole.
∞ Over one-half of original wetlands have been destroyed since European settlement.
∞ Out of the original 75 to 100 million acres of indigenous, woody-riparian habitat, 35 million acres remain.
∞ In response to flooding, since 1935 with the advent of major federal involvement:
o $25 billion has been spent on projects to “control” flooding.
o Federal and state agencies are spending in excess of $100 million annually on technical and planning assistance.
o Hundreds of millions of dollars are spent in most years on flood disaster relief and recovery assistance. …

How much of the above has to do with sea level rise? You have 50 states with their snouts in the flood trough.

JCH: As I understand correctly (I don’t have any references to cite), we don’t have many natural rivers anymore. Changes in river flooding are mostly caused by man, not climate change. Some factors include: increasing impermeable surfaces, cutting off flood plains with levees, changing were suspended sediments are deposited, etc. Environmentalists pushing to restore “natural rivers” are creating more flood risks on rivers that were changed by man a century ago.

Precipitation is supposed to rise only about 2%/K from rising GHGs, an increase that will be extremely useful (shouldn’t be dangerous) in a warmer world. Usually persistent weather caused by cut-off lows and other phenomena associated with undulations in the jet stream is not projected to increase according to AOGCMs, so the flooding associated with such persistence should not increase much. Flash flooding associated with short intense local downpours is expected to increase.

If pumping groundwater causes subsidence, can we promote rebound by pumping wastewater, CO2 (from carbon capture) or even salt water under vulnerable coastlines? Some locations in California’s Central Vally have subsided more than 5 m in the last century. Perhaps we could raise vulnerable coastlines by 1 m and get rid of unwanted materials at the same time.

We already do this and call it “enhanced oil recovery”, a process that must lead to rebound – or at least less subsidence. If subsidence isn’t associated with damaging earthquakes, rebound may not either. The possibility of a CO2 leak in a major urban area is frightening.

A lot depends on the structure of the aquifer. For fine sand, as in the Central Valley, subsidence is irreversible because the sand consolidates.

In the Central Valley, the typically slow process of draining fine-grained deposits has caused the permanent and irreversible consolidation of fine-grained subsurface deposits. This consolidation has resulted in extensive land subsidence, particularly in the San Joaquin Valley. Significant land subsidence (more than 1 foot) due to the withdrawal of groundwater has occurred in about half of the San Joaquin Valley, or about 5,200 mi^2. Small areas of the Sacramento Valley also have been affected by subsidence.

DeWitt: I remember reading about places in CA that had sunk substantially during the drought and have now clearly rebounded. So I believe rebound is possible in some places. Obviously you would only want to pump sea water well below any aquifers used by drinking water.

The issue of reversibility is an interesting one. If the pumping is going to lift the land, that could require extremely high pressure. However, if the pressure were caused by the weight of land above an aquifer, the water should geyser out of any well and not require pumping. It clearly wasn’t a carefully considered idea.

Perhaps future parts of cities by oceans will be floating. I think there are some plans, and they are not very expensive. Today many houses are jacked up when there is some ground subsidence. And we have probably some hundred years to do this job for most of the places, if it will be necessary at all. Why move people, when you can move houses? Just one method of mitigation.

Is there any economic advantage to having/building major cities on (ocean) coasts? Transportation is still important but how many of the 1.6 million people living on Manhattan Island or the 8.5 million people in greater New York City rely on or support ocean transportation?

[…] is expected to rise between around 0.3m to 0.6m (see Impacts – VI – Sea Level Rise 1 and IX – Sea Level 4 – Sinking Megacities) – this is from AR5 of the IPCC (under scenario RCP6). I mention this because the few people […]